Detecting actions in the fossil record is extremely tough. Unlike the animals themselves, which often have hard bits that fossilize, ancient actions can be discovered only in subtle ways.

Sex itself rarely leaves much behind for the ages. Sperm and shell-less eggs, all soft and squishy, do not fossilize, for example.

Yet John Long at Museum Victoria, Australia, and a team of researchers have found evidence for sex in the form of embryos inside fossils of placoderms—380-million-year-old armored fish that looked like swimming DustBusters.

Evidence for ancient underwater sex is not new—it had been found before in a small and obscure placoderm fish group called ptyctodontids.

But nobody knew if that meant that sex was unique to that group and that most other placoderms were fertilizing eggs outside of the body—or if it was a sign that sex was much more typical.

"We have now found that the major and most diverse group of placoderms, the arthrodires, were engaging in sexual intercourse. This kind of mating behavior was far more widespread than anyone realized," Long said.